Calcification in the bivalve periostracum

The periostracum in certain bivalves is imbedded with calcified, spiculelike structures analogous if not homologous to cuticular spicules found in the Aplacophora and Polyplacophora (chitons). Although rare or absent in most living bivalves, calcified periostracal structures are apparently an ancestral feature in some bivalve groups, i.e. the Mytilacea, Permophoridae, Myoida. and Anomalodesmata. Ancestors of the Bivalvia and Polyplacophora may have been covered with a flexible, spiculestudded cuticle. Shell plates in these two classes may have originated through a modification of the mechanism of spiculelike cuticular calcification. resulting in a primordial shell with simple prismatic structure.     

Diurnal Changes in Asparaginase Activity in Pea Leaves: II. REGULATION OF ACTIVITY

Pea leaf asparaginase is stabilized by asparagine and aspartateduring incubation. In crude extracts this effect was enhancedby products of the light reaction (NADPH, NADH, or reduced ferredoxin),but these compounds were ineffective on the purified enzyme,or in the absence of asparagine. MgATP, MgADP and oxidized ferredoxinreduced asparaginase activity in purified preparation reducedor oxidized glutathione had no effect. Asparaginase activitydoes not appear to be modulated via phosphorylation/dephosphorylation.The presence of calcium during extraction increased asparaginaseactivity more than 2-fold, but addition of calcium to extractsprepared in its absence had no effect; calmodulin had no effecton activity. Co-extraction of light- and dark-treated tissueshowed that soluble factors are not responsible for the diurnalvariation in asparaginase activity. Association of asparaginasewith membranes did not account for changes in extractable activity.Use of the protein synthesis inhibitors cycloheximide, puromycin,emetine, actinomycin D and cordycepin and the thiol proteaseinhibitor leupeptin suggested that mRNA and protein synthesisare required for the increase of asparaginase activity duringthe light period and that proteolytic degradation accounts forthe decrease during the dark. Key words: Pisum sativum, asparaginase, protein synthesis, proteolysis.     

One Molecule of Guanosine Triphosphate is Present in Each 30S Initiation Complex

FOR some time guanosine triphosphate (GTP) has been known to be involved in the initiation of bacterial protein synthesis^1–4, although its exact function remains obscure. Although GTP stimulates the binding of fMet-tRNA to 30S ribosomes^5,6, it does not seem to be hydrolysed in the process⁷. Hydrolysis is thought to occur later, either during or immediately after the junction of the 30S initiation complex (composed of a 30S ribosome, fMet-tRNA, mRNA, initiation factor F₁ and probably other components as we shall show; and hereafter referred to as 30S i-complex) with a 50S ribosome to form a 70S initiation complex^7–9. The use to which the energy released in this hydrolysis step is put is not yet known for certain.     

“Pleiotypic Response”

A hypothesis has been developed to relate stringent control in bacteria to a set of interactions involved in the regulation of growth of transformed and untransformed mammalian cells.     

Observations on Eimeria tenella in Feces of Inoculated Chickens

SYNOPSIS. In studies on coccidial excystation, Eimeria tenella sporulated oocysts were fed to 5 individually caged chickens, and during the next 4.5 hours, all droppings were collected immediately after excretion, mixed with 0.85% NaCl solution, then promptly examined for the parasites. Some samples were placed in cold storage at 4 C and examined at intervals thereafter. Three of the birds were necropsied after 5 hours and the intestinal contents examined. Apparently unbroken oocysts containing active sporozoites were in chicken droppings beginning 1–2.25 hours postinoculation; some activated forms were inside and others outside the sporocysts. Free sporozoites, sometimes numerous, first appeared in the feces 1.1–2 hours postinoculation. At necropsy, the state and condition of the parasites in the lumen of bird intestines were similar to those in its last fecal dropping. After being in cold storage for 48 hours, free sporozoites inside and outside the oocysts became active and moved about when placed on the warming stage of the microscope.     

H. A. GLEASON'S 'INDIVIDUALISTIC CONCEPT' AND THEORY OF ANIMAL COMMUNITIES: A CONTINUING CONTROVERSY

A tradition of natural history and of the lore of early twentieth-century ecology was that organisms lived together and interacted to form natural entities or communities. Before there was a recognizable science of ecology, Mobius (1877) had provided a name ‘biocoenosis’ for such entities. This concept persisted in the early decades of ecological science; at an extreme it was maintained that the community had integrating capabilities and organization like those of an individual organism, hence the term organismic community. In the 1950s- 1970s an alternative individualist concept, derived from the ideas of H. A. Gleason (1939), gained credence which held that communities were largely a coincidence of individualistic species characteristics, continuously varying environments and different probabilities of a species arriving on a given site. During the same period, however, a body of population based theory of animal communities became dominant which perpetuated the idea of patterns in nature based on biotic interactions among species resulting in integrated communities. This theory introduced an extended terminology and mathematical models to explain the organization of species into groups of compatible species governed by rules. In the late 1970s the premises and methods of the theory came under attack and a vigorous debate ensued. The alternatives proposed were, at an extreme, null models of random aggregations of species or stochastic, individualistic aggregations of species, sensu Gleason. Extended research and debate ensued during the 1980s resulting in an explosion of studies of animal communities and a plethora of symposia and volumes of collected works concerning the nature of animal communities. The inherent complexity of communities and the traditional differences among animal ecologists about how they should be defined and delimited, at what scale of taxa, space and time to study them, and appropriate methods of study and analysis have resulted in extended and as yet inconclusive discussions. Recent differences and discussions are considered under five general categories, evolution and community theory, individualistic concept, community definition, questions from community ecology and empirical studies. Communities are seen by some ecologists as entities of coevolving species and, in any case, it is necessary to integrate evolutionary ideas with the varied concepts of community. The individualistic concept of community, as a relative latecomer to discussions of animal community, is sometimes misconstrued as holding that communities are random assemblages of organisms without biotic interactions among species. Nevertheless, it has increasingly been accepted as supported by studies of diverse taxa and habitats. However, many other ecologists continue to argue for integrated, biotically controlled and evolved communities. Among the major difficulties of addressing the problems of community are problems of definition and terminology. One commentator noted that community ecology may be unique in the sciences because there is no consensus definition of community. One consequence of the lack of consensus definition is evident in the varied and diffuse questions posed in studies of community. Some critics of community ecology fault it for posing unanswerable questions. Recent empirical studies include various assessments about community ranging from deterministic, integrated and organismic to individualistic with various suggestions for compromise. The early emphasis on birds in studies of animal communities has expanded to obviate the argument that any position is constrained by the taxon studied. Insects, in general, are more prone to give rise to interpretation of a nonintegrated community. Parasite community studies have given rise to some distinctive categories and terminology. However, consensus is not achieved either within or among taxonomic groups or habitat groups. The extreme heterogeneity and complexity of communities (and of ecologists) has produced extended discussions of how to approach such multidimensional complexity. These discussions often turn on polarized positions of reductionism and experiment versus holism. Proponents of reductionism asserted that natural communities cannot be understood or their structure and organization predicted until experimental communities, or models thereof, are understood. Holists insisted that the inherent complexity and variability of communities cannot be elucidated in simplified experimental communities or in models. A more recent trend has urged pluralism, or, at least, mutual respect and dialogue, which are sometimes lacking, between proponents of these divergent approaches to communities. Recent work perpetuates the original dichotomy between integrated organismic community concept and individualistic non-integrated concept. The hope for a rule-governed community has extended to metarules and a new theory of community as divided into core species and satellite species is called into question. The problems of distinguishing between determinism and chance effects in community organization continue and the lost or fading hope of a general theory of community is revived in a search for rules that govern their assembly.